Cephalopoda
General characteristics of planktonic cephalopods

Early developmental stages of various coleoid cephalopods may be found in the plankton, including: squids (Teuthida), both neritic (Myopsida) and oceanic (Oegopsida); some cuttlefish (Sepiida: Spirula and some members of the family Sepiolidae); some benthic octopods of the genera Octopus, Scaeurgus, Euaxoctopus, Eledone (Octopoda, Octopodidae); and all developmental stages of Vampyromorpha and pelagic octopods (7 families of the Octopoda). Most adult squids belong to the nekton (some deepwater forms are semiplanktonic/seminektonic), cuttlefish to the nekton or nekto-benthos, and octopodid octopuses to the benthos.

Newborn cephalopods are termed either "juveniles", if they resemble small adults, or "paralarvae" (in malacologists’ jargon) or "larvae" (in ichthyologists’ jargon), if they are different from the adults and have some larval features. The terms "paralarva" and "larva" are equivalent, malacologists using "paralarva" simply to distinguish cephalopods from those molluscs that undergo full metamorphosis (Nesis, 1979b; Young and Harman, 1988). I will use the term larva in the same sense as it is used in ichthyology.

Identification of larvae and early juveniles to species level is difficult, or even impossible in the case of very early larvae, because species-specific characters usually develop only in the postlarval stage. The use of identification keys for adults is impossible, and keys specifically for planktonic stages are problematic because taxonomically important characters develop in sequence. Thus, different keys would be required for representatives of the same species depending on their size. Usually, precise identification to only the generic level is possible.